Is fluoride in water toxic to babies’ brains? A rigorous study raises alarms.

A new study that suggests fluoride is harmful to developing brains is likely to fan the smoldering debate over the safety of adding the tooth-protecting mineral to public drinking water.

The study by Canadian researchers, published Monday in JAMA Pediatrics, found that pregnant women who drank more fluoridated water had children with lower intelligence scores at ages 3 and 4. An increase of a milligram of fluoride per day — the amount in about five cups of water — was linked to a loss of 3.7 IQ points for boys and girls.

However, the researchers separately analyzed the amount of fluoride in the pregnant women’s urine rather than just their water intake. In that sample, they found higher levels were linked to lower IQ only in boys.

The idea that fluoride is a brain toxin “must now be given serious consideration,” David C. Bellinger, a neurologist at Boston Children’s Hospital and Harvard Medical School, wrote in an editorial. “If the hypothesis is true, the implications are worrisome.”

Anticipating heat, journal editor Dimitri A. Christakis took the usual step of including a note defending the decision to publish the article, which was subjected to extra scrutiny: “The mission of the journal is to ensure that child health is optimized by bringing the best available evidence to the fore … regardless of how contentious the results may be.”

A representative of the American Academy of Pediatrics said it continues to recommend that children drink fluoridated water. Studies have linked poor oral health to poor school performance, among other problems.

“Tooth decay remains a significant, often lifelong, health problem for children and families,” pediatrician Aparna Bole, an executive committee member of the group’s Council on Environmental Health, said in an email. “This study is thought-provoking, but the fact that results were different for boys and girls make them somewhat difficult to interpret, and fluoride intake in children was not examined. We support the public health sector’s continual evaluation of optimal community water fluoridation.”

Speaking for the American Dental Association, Brittany Seymour said one study is not enough to alter public health policy, especially such a well-researched one. The Harvard School of Dental Medicine assistant professor then added her personal view.

“I’m passionate about teeth, but my most important job is as a mom. I’m raising my daughter in Boston," she said of her 6-year-old. "Boston is fluoridated. I haven’t once felt concern that she’s drinking fluoridated water. In fact, I would be much more concerned if she wasn’t. This study doesn’t change my comfort level with my own daughter continuing to drink fluoridated water.”

Since the mid-1940s, fluoride has been added to U.S. drinking water to prevent tooth decay. More than 70% of Americans now have fluoridated water. The U.S. Centers for Disease Control and Prevention (CDC) calls it one of the 10 top public health achievements of the 20th century.

The new study does not appear likely to change that.

"Community water fluoridation is one of the most practical, cost-effective, equitable, and safe measures communities can take to prevent tooth decay and improve their oral health, and is the best method of delivering fluoride to all members of the community, regardless of age, education, income level, or access to routine dental care,” said a CDC spokeswoman.

Many other studies over nearly 75 years have attested to the benefits of fluoride and deemed it safe, and it has been recommended by nearly all public health, medical, and dental organizations, she added.

Despite wide adoption, fluoridation remains controversial, with opponents arguing that the health effects aren’t completely understood, and it amounts to a compulsory medication. In this region, the Philadelphia Water Department adds fluoride to residents’ water, but Aqua Pennsylvania, which serves 1.4 million customers in four suburban counties, does not. New Jersey American Water defers to a mix of community preferences.

Rates of tooth decay have declined in the U.S., but not more than in Europe, where people rely on fluoride-containing dental products and supplements.

In 2015, the U.S. Department of Health and Human Services cut the recommended amount of fluoride in drinking water because many children were getting too much, causing white patches called fluorosis on their teeth. The new standard was set at 0.7 milligrams per liter, rather than a range of 0.7 to 1.2 milligrams.

A number of studies in lab animals, and population-based studies, have suggested that high fluoride levels may have harmful neurological effects. The research has been largely dismissed as preliminary or of low quality.

In 2017, a more rigorous study grabbed attention. An international research team measured fluoride in archived urine samples taken from pregnant women and their offspring in Mexico. Mothers with higher fluoride levels had children who performed worse on thinking tests at age 4, and ages 6 to 12.

The new study, led by Rivka Green, a clinical neuropsychology doctoral candidate at York University in Toronto, used archived urine samples and data on water, tea, and coffee consumption that was collected from Canadian women as part of previous research on environmental chemicals.

The new analysis compared 228 mother-child pairs who lived in nonfluoridated areas — which make up almost 60% of Canada — with 141 pairs who lived in fluoridated areas between 2008 and 2011.

The IQ losses “were observed at fluoride levels typically found in white North American women,” the authors wrote. “This indicates the possible need to reduce fluoride intake during pregnancy.”

The researchers could not explain why higher urine fluoride levels were linked to lower IQ in boys, but not girls. They speculated, however, that boys’ brains and nerves may be more vulnerable.

“We know boys have higher rates of autism, attention deficit hyperactivity disorder, and learning disabilities,” Green said. “So it could be they are more susceptible to certain contaminants during prenatal development.”

Philippe Grandjean, a Danish researcher and adjunct professor of environmental health at Harvard University, has long been involved in scientific efforts to warn of the brain development hazards of lead, mercury, and now fluoride. He led a 2012 review of 27 fluoride studies, most from China, that bolstered the neurotoxin theory. The review was later attacked as flawed.

“There’s so much investment in promoting water fluoridation, and therefore so much face-losing that is at stake,” he said. “No one wants to admit failure. I’m sure that’s true of dentists and the CDC. But I think the time has come for us to ask the CDC to reconsider fluoridation, because how many millions of children do we want to potentially put at risk of a small loss of IQ?”